Ignition System: Description and Operation
IGNITION SYSTEMNOTE: The engine use a fixed ignition timing system. Basic ignition timing is not adjustable. All spark advance is determined by the Powertrain Control Module (PCM).
The engine uses a distributor, crankshaft sensor and ignition coil. The system's main components are the distributor, distributor pickup, camshaft signal, crankshaft signal and ignition coil.
The crankshaft position sensor and camshaft position sensor are hall effect devices. The camshaft position sensor and crankshaft position sensor generate pulses that are inputs to the PCM. The PCM determines crankshaft position from these sensors. The PCM calculates injector sequence and ignition timing from crankshaft position. For a description of both sensors, refer to Camshaft Position Sensor and Crankshaft Position Sensor.
SPARK PLUGS RESISTER
All engines use resistor spark plugs. They have resistance values ranging from 6,000 to 20,000 ohms when checked with at least a 1000 volt spark plug tester.
Do not use an ohm meter to cheek the resistance of the spark plugs. This will give an inaccurate reading.
Refer to the Specifications for gap and type of spark plug.
SPARK PLUGS PLATINUM
The engine utilize platinum spark plugs. Refer to the maintenance schedule.
All engines use resistor spark plugs. They have resistance values ranging from 6,000 to 20,000 ohms when checked with at least a 1000 volt spark plug tester.
Do not use an ohm meter to check the resistance of the spark plugs. This will give an inaccurate reading.
Platinum Pads:
The spark plugs are double platinum and have a recommended service life of 100,000 miles for normal driving conditions per schedule A. The spark plugs have a recommended service life of 75,000 miles for severe driving conditions per schedule B. A thin platinum pad is welded to both electrode ends as show in. Extreme care must be used to prevent spark plug cross threading, mis-gaping and ceramic insulator damage during plug removal and installation.
CAUTION: Cleaning of the platinum plug may damage the platinum tip.
SPARK PLUG CABLE
Spark Plug cables are sometimes referred to as secondary ignition wires. The wires transfer electrical current from the ignition coil pack, distributor, to individual spark plugs at each cylinder. The resistive spark plug cables are of nonmetallic construction. The cables provide suppression of radio frequency emissions from the ignition system.
Check the spark plug cable connections for good contact at the coil, distributor cap towers, and spark plugs. Terminals should be fully seated. The insulators should be in good condition and should fit tightly on the coil, distributor and spark plugs. Spark plug cables with insulators that are cracked or torn must be replaced.
Clean Spark Plug cables with a cloth moistened with a non-flammable solvent. Wipe the cables dry. Check for brittle or cracked insulation.
IGNITION COIL
WARNING: THE DIRECT IGNITION SYSTEM GENERATES APPROXIMATELY 40,000 VOLTS. PERSONAL INJURY COULD RESULT FROM CONTACT WITH THIS SYSTEM.
Ignition Coil Pack:
The ignition coil assembly consists of 3 independent coils molded together. The coil assembly is mounted on the intake manifold. Spark plug cables route to each cylinder from the coil.
The coil fires two spark plugs every power stroke. One plug is the cylinder under compression, the other cylinder fires on the exhaust stroke. The Powertrain Control Module (PCM) determines which of the coils to charge and fire at the correct time.
Coil 1 fires cylinders 1 and 4, coil 2 fires cylinders 2 and 5, coil 3 fires cylinders 3 and 6.
The Auto Shutdown (ASD) relay provides battery voltage to the ignition coil. The PCM provides a ground contact (circuit) for energizing the coil. When the PCM breaks the contact, the energy in the coil primary transfers to the secondary causing the spark. The PCM will de-energize the ASD relay if it does not receive the crankshaft position sensor and camshaft position sensor inputs. Refer to Auto Shutdown (ASD) Relay-PCM Output, for relay operation.
Power Distribution Center (PDC), Powertrain Control Module (PCM):
AUTOMATIC SHUTDOWN (ASD) RELAY
The ASD relay is located in the Power Distribution Center (PDC). The PDC is located on the driver's side inner fender well. A label on the underside of the PDC cover identifies the relays and fuses in the PDC.
The Powertrain Control Module (PCM) operates the Auto Shutdown (ASD) relay by switching the ground path ON and OFF.
The ASD relay supplies battery voltage to the fuel injectors, electronic ignition coil and the heating elements in the oxygen sensors.
The PCM controls the relay by switching the ground path for the coil side of the relay ON and OFF. The PCM turns the ground path OFF when the ignition switch is in the Off position unless the O2 Heater Monitor test is being run. When the ignition switch is in the On or Crank position, the PCM monitors the crankshaft position sensor and camshaft position sensor signals to determine engine speed and ignition timing (coil dwell). If the PCM does not receive the crankshaft position sensor and camshaft position sensor signals when the ignition switch is in the Run position, it will de-energize the ASD relay.
CRANKSHAFT POSITION SENSOR
Crankshaft Position Sensor:
The crankshaft sensor is located on the passengers side of the transmission housing, above the differential housing. The bottom of the sensor is positioned next to the drive plate.
Crankshaft Position Sensor:
Timing Slots:
The crankshaft position sensor detects slots cut into the transmission driveplate extension. There are 3 sets of slots. Each set contains 4 slots, for a total of 12 slots. Basic timing is set by the position of the last slot in each group. Once the Powertrain Control Module (PCM) senses the last slot, it determines crankshaft position (which piston will next be at TDC) from the camshaft position sensor input. The 4 pulses generated by the crankshaft position sensor represent the 69 °C, 49 °C, 29 °C, and 9 °C BTDC marks. It may take the PCM one engine revolution to determine crankshaft position.
The PCM uses crankshaft position reference to determine injector sequence, ignition timing and the presence of misfire. Once the PCM determines crankshaft position, it begins energizing the injectors in sequence.
CAMSHAFT POSITION SENSOR
Camshaft Position Sensor Location:
The camshaft position sensor is mounted in the front of the timing case cover.
Camshaft Position Sensor:
The camshaft position sensor provides cylinder identification to the Powertrain Control Module (PCM). The sensor generates pulses as groups of notches on the camshaft sprocket pass underneath it. The PCM keeps track of crankshaft rotation and identifies each cylinder by the pulses generated by the notches on the camshaft sprocket. Four crankshaft pulses follow each group of camshaft pulses.
Camshaft Sprocket:
When the PCM receives 2 cam pulses followed by the long flat spot on the camshaft sprocket, it knows that the crankshaft timing marks for cylinder 1 are next (on driveplate). When the PCM receives one camshaft pulse after the long flat spot on the sprocket, cylinder number 2 crankshaft timing marks are next. After 3 camshaft pulses, the PCM knows cylinder 4 crankshaft timing marks follow. One camshaft pulse after the 3 pulses indicates cylinder 5. The 2 camshaft pulses after cylinder 5 signals cylinder 6. The PCM can synchronize on cylinders 1 or 4.
When metal aligns with the sensor, voltage goes low (less than 0.3 volts). When a notch aligns with the sensor, voltage switches high (5.0 volts). As a group of notches pass under the sensor, the voltage switches from low (metal) to high (notch) then back to low. The number of notches determine the amount of pulses. If available, an oscilloscope can display the square wave patterns of each timing event.
Top Dead Center (TDC) does not occur when notches on the camshaft sprocket pass below the cylinder. TDC occurs after the camshaft pulse (or pulses) and after the 4 crankshaft pulses associated with the particular cylinder. The arrows and cylinder call outs on Figure 4 represent which cylinder the flat spot and notches identify, they do not indicate TDC position.
KNOCK SENSOR
The knock sensor threads into the cylinder block. The knock sensor is designed to detect engine vibration that is caused by detonation.
When the knock sensor detects a knock in one of the cylinders, it sends an input signal to the PCM. In response, the PCM retards ignition timing for all cylinders by a scheduled amount.
Knock sensors contain a piezoelectric material which constantly vibrates and sends an input voltage (signal) to the PCM while the engine operates. As the intensity of the crystal's vibration increases, the knock sensor output voltage also increases.
The voltage signal produced by the knock sensor increases with the amplitude of vibration. The PCM receives as an input the knock sensor voltage signal. If the signal rises above a predetermined level, the PCM will store that value in memory and retard ignition timing to reduce engine knock. If the knock sensor voltage exceeds a preset value, the PCM retards ignition timing for all cylinders. It is not a selective cylinder retard.
The PCM ignores knock sensor input during engine idle conditions. Once the engine speed exceeds a specified value, knock retard is allowed.
Knock retard uses its own short term and long term memory program.
Long term memory stores previous detonation information in its battery-backed RAM. The maximum authority that long term memory has over timing retard can be calibrated.
Short term memory is allowed to retard timing up to a preset amount under all operating conditions (as long as rpm is above the minimum rpm) except WOT. The PCM, using short term memory, can respond quickly to retard timing when engine knock is detected. Short term memory is lost any time the ignition key is turned OFF.
NOTE: Over or under tightening affects knock sensor performance, possibly causing improper spark control.